Free electron laser orbital debris removal system

ABSTRACT

Orbital debris removal (ODR) systems under the present approach may use a ground- or surface-based FEL and mirror system with sufficient power and both spatial and temporal resolution to both locate Category II OD (1 cm to 10 cm diameter) in low Earth orbit (LEO, 160 to 2000 km altitude) and remove these objects from orbit. Locating the Category II OD is performed by having the light beam from an FEL and its beam director scan a volume of space of interest and then observing the light reflected from the OD. Removing the OD may include heating the OD to a sufficiently high temperature to evaporate the OD, changing the orbit of the OD such as to lower the perigee, or both. Megawatt-class MOPA FELs for, inter alia, removing OD, are described.

CROSS REFERENCE TO RELATED APPLICATIONS

-   -   This application is the U.S. national phase of International        Application No. PCT/US2018/064398 filed Dec. 7, 2018, which        claims to the benefit of U.S. Provisional Patent Application No.        62/596,499, filed Dec. 8, 2017, U.S. Provisional Patent        Application No. 62/680,858, filed Jun. 5, 2018, and U.S.        Provisional Patent Application No. 62/724,893, filed Aug.        30, 2018. The contents of each are expressly incorporated by        reference in their entirety.

STATEMENT REGARDING GOVERNMENT SUPPORT

None.

FIELD OF THE INVENTION

The present disclosure relates to free electron lasers and the removalof orbital debris with free electron lasers.

BACKGROUND—INTRODUCTION

Orbital debris removal (ODR) by a variety of high power lasers has beenconsidered for several decades. Proposed embodiments typically have thelasers in orbit or on the ground, and a few embodiments have consideredutilizing orbiting relay mirrors to deliver the laser power. Most of thelasers suggested utilize high flux pulses, typically less than 10nanoseconds in length, that deliver sufficient flux to the surface ofthe orbital debris (OD) that some of the debris evaporates from thesurface thereby providing pulses of thrust to the OD. Multiple pulsesare utilized to sufficiently alter the OD's orbit that the OD is burnedup in the atmosphere on subsequent orbit, possibly years later. Freeelectron lasers (FELs) have been considered as one of the options forlasers. For example, in the mid 1990s NASA considered them with projectOrion, but consideration of FELs has been much less than other forms oflasers because FELs until now have had insufficient average power and/orinsufficient pulsed power for most options and the innovations requiredfor successful implementation were not developed. For example, NASA'sproject Orion and similar projects did not appreciate how thecombination of average power, pulsed power, frequency and time controlcould be used to locate OD and then remove the OD. If anything, theconventional art discounted FELs for ODR.

One of the biggest challenges for ODR is the ability to locate CategoryII OD with diameters in the 1 to 10 cm range. (Category I is OD<1 cm andCategory III is OD>10 cm.) OD with diameters greater than approximately10 cm can be located and tracked by ground- or surface-based radar. Withapproximately 400,000 Category II OD items in low earth orbit (LEO),defined as 160 to 2000 km in altitude and with a large number ofsatellites operating in the LEO range, there is great interest infinding an efficient solution to both locate the Category II OD itemsand remove them. Prior ODR solutions have been ineffective at dealingwith Category II OD.

Very high average power FELs, i.e., generally average power (sometimesreferred to as CW power for continuous wave power) in excess of about 10kW, have been demonstrated only recently, at the U.S. Department ofEnergy's Thomas Jefferson National Accelerator Facility (Jefferson Lab),which produced a beam of 14 kW at a 1.6 micron wavelength in 2006.Jefferson Lab achieved this performance level using an electron beamwith an average current of 10 mA resulting in a performance of 1.4 kWaverage photon beam power per mA of average beam current. No very highaverage power FEL has surpassed the 14 kW average photon beam power thatJefferson Lab achieved. Higher average power photon beams are producedby multiple versions of solid state and gas lasers. However, theselasers typically do not have the diffraction limited beam quality thatan FEL output photon beam can achieve, and they typically only operateat a narrow range of wavelengths or a single wavelength (where FELs canoperate over broad ranges of wavelengths).

High capital cost and operating expenses of prior technologies havelimited the deployment of FELs. Examples include: the need to havesignificant amounts radio frequency (RF) capacity installed to providethe power for the very high levels of electron beam current required andto deal with microphonics in the cryogenic components that arecompensated for by extra installed RF; the need to operate at 2 K,superfluid Helium temperature, rather than 4 K, atmospheric pressureliquid helium temperature, where refrigeration is much less expensive;and the need to manage waste heat from coherent synchrotron radiation(CSR) and transition radiation (TR) due to the very high levels ofelectron beam current projected to be required to achieve high averagephoton beam power.

Most FEL development for over a decade has been focused on x-ray FELswith the goal of producing very high peak power, e.g. above a terawatt,for very short durations, e.g. below a picosecond, for scientificexperiments. The XFEL is now turning on at DESY, in Germany and theLCLS-II is under construction at SLAC at Stanford, Calif. A number ofthe enhancements needed for the very high peak power x-rays could inprinciple be applied to very high average power FELs, but theseinnovations and developments have not been done for very high averagepower.

What is needed, then, is a very high average beam power FEL, capable ofoperating at wide wavelength ranges with controllable beam structure foruse in ODR systems and other very high average beam power applications.

SUMMARY

This disclosure relates to FEL ODR systems and methods, such as ground-or surface-based FEL and mirror systems and methods with sufficientpower and spatial and temporal resolution, to both locate Category II ODand remove such objects from orbit. Embodiments of the system can alsoremove Category I OD and some smaller Category III OD, though theembodiments described herein are generally optimized for Category II OD.Locating the Category II OD under the present approach may be performedby having the light beam from an FEL and its beam director scan a volumeof space of interest, and then observing the light reflected from theOD. FELs can emit a stream of sub-picosecond pulses at any repetitionrate up to the radio frequency (RF) that is utilized to power theelectron bunches in the FEL. The pattern of the light pulses and thetravel time of the pulses and their reflections may then be used in thepresent approach to optimize signal-to-noise in detecting the lightreflecting from the OD. For example, the pattern of light pulses, and/orthe frequency modulation of the light beam, may be varied in thetransmitted light. A separate mirror may be utilized to detect the lightreflected from the OD, and may detect the pattern variations foradditional improvement in the signal-to-noise ratio.

The present approach also provides for removal of OD. Generally, theprocess of OD removal involves heating the OD to a sufficiently hightemperature to evaporate the OD, preferentially heating the Earth-facingside of OD even while rotating, such that the evaporation from theEarth-facing side provides thrust on the OD. The thrust changes theorbit of the OD and lowers the perigee. Some embodiments of the presentapproach may include combinations of these aspects. The optics of beamdirector mirror(s) may be arranged such that the apparatus can switchfrom a scanning mode to directed energy (DE) mode, in which the lightbeam is concentrated on an instance of OD.

This disclosure also relates to an optimized MOPA FEL system thatgenerates very high average power (i.e., average power above about 10kW), and therefore are ideal for ODR applications. Embodiments mayinclude an electron source with an average electron current below about1 mA per 10 kW of average photon beam power, as well as an electronbooster, an electron accelerator, an electron beam transport system, twoundulators, and a photon beam, in a MOPA FEL configuration. A singleelectron beam with at least dual energies may be used to feed both anundulator in an OSC configuration and an undulator in an AMPconfiguration. Alternatively, correlated energy spread from injectortiming manipulation or subharmonic cavities can be utilized, and as afurther alternative the initial electron beam may be differentiallyamplified to separate it into two beams that separately feed the OSC andAMP undulators and are then recombined. The AMP undulator may be atapered undulator. In some embodiments, a tapered undulator may providefor extreme transverse compression of the electron beam from a strongFODO lattice. An electron beam stop may be included to provide energyrecovery of the post undulator electron beam. The resultant performancein terms of average photon beam power per unit of average electron beamcurrent is more than a factor of 10 above what has been achieved todate, and the performance can reach 30 times and above what has beenachieved to date. In addition, the efficiency for converting electricalpower to photon power is a factor of 3 to 10 higher than whatcontemporary technologies have achieved to date. The present approachadvantageously provides for a combination of the lower beam currentsrequired and improvements in vibration management at cryogenictemperatures that drive the amount of RF required. Additionalimprovements may be achieved by operating the cryogenic system at 4 Krather than 2 K, and from utilization of advanced beam stop technology.

Embodiments of the present approach may take the form of methods forfree electron laser (FEL) orbital debris removal. A light beam may beemitted from an FEL to a beam director. The light beam may have lightpulses and a diffraction-limited portion. Either or both of the lightpulse pattern and the frequency modulation of the light beam may bevaried, such as to improve the signal-to-noise ratio in the subsequentdetection of reflected light. The light beam may be directed from thebeam director, such as a mirror, to a search region, such as a region ofspace to be scanned for orbital debris. Light reflected from orbitaldebris may be detected at an observation mirror. When the emitted lightincludes variations, the reflected light will include at least one of areflected light pulse pattern and a reflected modulated frequency.Orbital debris may be identified in the search region, such as throughdetection and analysis of the reflected light. For example, reflectedlight pulse patterns and/or reflected modulated frequencies may be usedin the analysis of reflected light to improve the signal-to-noise ratio,and identify orbital debris in the search region. One or more orbitalparameters for the orbital debris may be determined, and the light beammay be redirected to the orbital debris for removal. For example, insome embodiments the emitted light beam may be focused for a narrowfield projection, such as to specifically target the orbital debris.

Some embodiments may include tracking the identified orbital debris witha broad field projection light beam. In some embodiments, the emittedlight beam may be a broad beam having a cone opening angle of at least0.05 milliradians and less than 2.0 milliradians, and more preferablyand more preferably from about 0.3 to about 1.0 milliradians. However,the opening angle for broad field projection depends on the elevationbeing scanned, and unless otherwise stated the present approach is notlimited to a specific opening angle in the broad field projection forscanning and locating OD.

In some embodiments, directing the light beam at a search region may beperformed by, for example, using a first mirror having a broad fieldprojection for scanning and tracking, and re-directing the light beam atthe orbital debris may be performed by using a second mirror.Re-directing the light beam at the orbital debris in some embodimentsmay involve focusing a waist of the diffraction-limited portion of thelight beam at the orbital debris. In some embodiments, switching betweenthe first mirror and the second mirror may be performed at a ratesufficient to maintain the OD within a field of projection of the lightbeam.

In some embodiments of the present approach, the FEL is a MOPA FEL.Embodiments of a MOPA FEL may include, for example, an electron source,such as an electron source with an average electron beam current belowabout 2 mA and above 1 microamp per 10 kW of average photon beam power,and the electron source may preferably have an emittance less than 4 mmmrad and above 1 micrometer mrad, an energy spread less than one part inone hundred. The MOPA FEL may also include an electron booster, anelectron accelerator, an OSC undulator, and an AMP undulator. The MOPAFEL may in some embodiments include one or more of a boron nitridenanotube (BNNT) cryosorber and a BNNT vibration damper. It should beappreciated that various AMP and OSC undulators may be used, dependingon the embodiment, for example, the AMP undulator may be a taperedundulator or a non-tapered undulator. In some embodiments, the OSCundulator and/or the AMP undulator may be one of a planar undulator anda helical undulator. The AMP undulator may be, in some embodiments, aplanar undulator having a non-tapered portion. In some embodiments, theOSC undulator and/or the AMP undulator is a helical undulator. It shouldbe appreciated that various combinations of undulators may be used,apart from the specific combinations disclosed herein. In someembodiments, the MOPA FEL includes SRF cavities incorporating a boronnitride nanotube vibration damper.

In some embodiments, the MOPA FEL is a Compact MOPA FEL. Light may begenerated from a Compact MOPA FEL by, for example, splitting anaccelerated into a first split electron beam and a second split electronbeam, passing the first split electron beam through an AMP undulator,passing the second split electron beam through an OSC undulatorgenerating an OSC light beam, amplifying the OSC light beam in the AMPundulator to generate the emitted light beam, and passing the firstsplit electron beam after the AMP undulator and the second electronsplit beam after the OSC undulator through a combiner, deacceleratingthe first split electron beam and accelerating the second split electronbeam in the secondary accelerator to form a combined electron beam, andpassing the combined electron beam through the primary accelerator forenergy recovery.

Embodiments of the present approach may take the form of FEL orbitaldebris removal systems. Systems may include, for example, a MOPA FEL,preferably having average power above about 10 kW and configured to emita light beam having light pulses. At least a portion of the light beammay be a diffraction-limited portion. Embodiments may further include alight pulse pattern and frequency modulation controller, a beam directorconfigured to receive the emitted light beam and re-direct the emittedlight beam to a search region, a scanning mirror with a broad fieldprojection and a removal mirror having a narrow field projection, anobservation mirror configured to detect a reflected light beam from thesearch region, and an orbital debris identification controllerconfigured to identify an orbital debris in the search region from atleast one of the reflected light pulse pattern and reflected modulatedfrequency, and provide at least one scanning mirror orbital debrisorbital parameter. The reflected light beam may have, for example, areflected light pulse pattern and reflected modulated frequency, whichmay be used to improve signal-to-noise ratio. In some embodiments, thebeam director may be configured to switch between the scanning mirrorand the removal mirror. Some embodiments of an FEL orbital debrisremoval system may include a MOPA FEL with an average beam power of atleast about 100 kW, and in some embodiments the MOPA FEL may include anaverage power of at least about 1 MW.

Some embodiments of the FEL orbital debris removal system include aCompact MOPA FEL. Embodiments of a Compact MOPA FEL may include, forexample, an electron beam splitter directing a first split electron beamthrough an AMP undulator and a second split electron beam through an OSCundulator, and an electron beam combiner after the AMP undulator and theOSC undulator with an output electron beam arc to a primary acceleratorfor energy recovery. The OSC undulator may have an output light beampath feeding an input light beam path of the AMP undulator. In someembodiments, the FEL has at least one of a BNNT cryosorber and a BNNTvibration damper.

Some embodiments of the present approach may take the form of a masteroscillator/power amplifier (MOPA) free electron laser (FEL). In someembodiments, the FEL has an electron source with an average electronbeam current below about 2 mA and above 1 microamp per 10 kW of averagephoton beam power, and in some embodiments the electron source has anemittance less than 4 mm mrad and above 1 micro m mrad, an energy spreadless than one part in one hundred. Embodiments of an FEL may include anelectron booster, an electron accelerator, an OSC undulator having anoutput light beam path, and an AMP undulator with an input light beampath connected to the OSC undulator output light beam path, and anoutput emitted light beam path. In some embodiments, the FEL has atleast one of a BNNT cryosorber and a BNNT vibration damper.

Embodiments of a MOPA FEL may include different combinations ofundulators. For example, an AMP undulator may be a tapered undulatorand/or a non-tapered undulator. The OSC undulator and/or the AMPundulator may be a planar undulator. In some embodiments, the AMPundulator may be a planar undulator with a non-tapered portion. In someembodiments, the OSC undulator and/or the AMP undulator may be one of aplanar undulator and a helical undulator. The present approach is notintended to be limited to the combinations of undulators set forth inthe specific examples disclosed herein. Some embodiments may include oneor more SRF cavities having a BNNT vibration damper.

In some embodiments, the FEL may include an electron beam splitter witha first split electron beam arc connected to the AMP undulator and asecond split electron beam arc connected to the OSC undulator, and anelectron beam combiner after the AMP undulator and the OSC undulatorwith an output electron beam arc connected to a primary accelerator.Such configurations may be especially useful for energy recovery. Insome embodiments, the OSC undulator includes an output light beam pathfeeding into the AMP undulator, which in turn emits a light beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of the present approach in which FELlocates OD by beaming light into a scanned region of space, and thesystem detects reflected light.

FIG. 2 illustrates FEL removal of OD by beaming light onto the OD andmonitoring the reflected light according to an embodiment of the presentapproach.

FIG. 3 illustrates an embodiment of a MOPA FEL with Parallel AxesPhotons with Energy Recovered Circulating Linac at Intense Powers(PAPERCLIP) architecture.

FIG. 4 illustrates the different length electron bunch trainexperiencing different accelerations and deaccelerations in the linac(linear accelerator).

FIG. 5 illustrates the overlaps of the optical and electron beam bunchesin the AMP undulator.

FIG. 6 illustrates the energy distribution of electrons following theAMP undulator.

FIG. 7 illustrates megawatt-class performance over a range ofparameters.

FIG. 8 illustrates an alternate embodiment of a MOPA FEL.

FIG. 9 illustrates the beam pulse pattern for the alternate embodiment.

DETAILED DESCRIPTION

Described herein are embodiments of FEL ODR systems and methods capableof average power, peak power light pulses, and temporal and spatialresolution combinations, to both locate and remove OD, and particularlyCategory II OD in some embodiments. Category II OD typically hasdiameters in the 1 to 10 cm range. Category I is OD<1 cm and CategoryIII is OD>10 cm. As will be appreciated by those of ordinary skill inthe art, embodiments of an FEL ODR system under the present approach canalso remove OD in Category I and Category III. A variety of FEL formatsare possible, including Oscillator (OSC) FEL as used for the JeffersonLab 14 kW FEL, Self-Amplified Spontaneous Emission (SASE) FEL whichoperates as an Amplifier (AMP) of noise, Regenerative Amplifier FEL(RAFEL), Master Oscillator/Power Amplifier (MOPA) FEL, and opticalklystron FEL.

FIG. 1 illustrates an embodiment of an FEL ODR system 10 located on thesurface of the Earth 11. The system may be used to locate OD 16. The FEL12 is, in this embodiment, a megawatt class FEL so as to have sufficientpower to fill a scanned region of space with enough light to detect theOD 16. Megawatt (MW) class generally means that the FEL can produce atleast about 1 MW of average power or more for periods of at least aboutone second, where average power will be considered the total energydelivered for a period of one second. In preferred embodiments of thepresent approach, the total period of 1 MW average power performancelasts for at least hours, and more preferably the period lasts for days,and preferably with only brief, e.g. less than about 30 minutes, timewith the FEL 12 off during the performance period. It should beappreciated that those of ordinary skill in the art may select the FELclass appropriate for a particular embodiment and may deviate frompreferred embodiments described herein. Novel methods to create megawattclass FEL suitable for preferred embodiments are described in below andin the incorporated references. The diffraction limited light beam 13from the FEL 12 is directed to the beam director 14. Beam director 14may include one or more mirrors transporting the light beam 13 to aprimary mirror beam director (not shown), as may be known in the art.While in principle beam director 14 can direct the light beam 13 in anydirection, some embodiments of the present approach configured for ODRfrom the surface Earth 11 preferably cover a cone of about 20°, and insome embodiments up to about 45°, in all directions with respect to thezenith. Embodiments may deviate from these directions, as needed.

A demonstrative ODR embodiment with the parameters in Table I will beutilized for an illustrative example. The parameters are illustrativeonly and it should be appreciated that a wide range of parameters thatutilize the characteristics of this FEL based ODR system can be used.

TABLE 1 Illustrative FEL ODR Parameters FEL power 1 MW FEL light beamwavelength 400 nm FEL RF frequency 750 MHz FEL electron beam emittance 1mm-mrad Primary mirror diameter 3 m Peak elevation of OD 800 km Diameterof scanned region at Peak 0.5 km Opening angle of broad beam cone 0.6milliradians OD diameter 2 cm OD reflectivity 0.2 OD average emissivity0.2 OD rotation 50 % Atmospheric transmission 80 % Observation mirrordiameter 10 m Mirrors' angular resolutions 0.3 micro-rad Category II ODobjects 400,000 in LEO Goal: min Category II OD removed 20,000objects/year

During operation, light beam 15 from the beam director 14 expands tocreate a broad beam to fill a broad scanned region of spaceapproximately in the shape of a cone above the beam director 14 thatcreates a broad scanning and tracking region. As used in thisdescription, a broad field/beam typically has the opening angle of thecone from 0.1-1.0 milliradians, though in some embodiments the openingangle of the cone may be beyond this range. For example, the broad fieldopening angle may be from about 0.05 to about 2.0 milliradians, and morepreferably from about 0.3 to about 1.0 milliradians. However, it shouldbe appreciated that the opening angle for broad field projection isnecessarily dependent upon the target elevation being scanned, and thepresent approach is therefore not limited to a specific opening angle inthe broad field projection for scanning and locating OD. The cone shapedepends on the particular embodiment and may be an oval or similarshape. Using the exemplar values in Table 1 and the length of shortesttrack of a piece of OD traversing the scanned region at approximately70% of the diameter of the scanned region, the area being scanned at apeak elevation of 800 km is 0.15 km² and the cone opening angle is near0.6 milliradians. In some embodiments of the present approach, the fullaltitude range being scanned is from 160 km to 2000 km. The total LEOvolume is 1.29E12 km³ so the average Category II OD density is 3.1E-7Category II OD/km³. The density of OD is non-uniform with altitude witha peak near 800 km altitude and a much lower secondary peak near 1,450km altitude. At present, over two-thirds of the Category II OD isbetween 600 km and 1,000 km. For LEO, there is relatively little ODbelow 400 km and relatively little above 1,600 km. OD at 800 km takes100.87 minutes to orbit Earth at a speed of 7.45 km/s. This results in apiece of OD taking about 58 milliseconds (or slightly longer) to transitthe cone being scanned at the 800 km altitude.

Embodiments of an FEL 12 according to the present approach may beconfigured to produce sub-picosecond long light bunches by utilizingelectron bunches that can be at any rate and distribution up to full RFfrequency of the FEL 12. In the illustrative embodiment, this is up to750 MHz, but those having ordinary skill in the art will appreciate thatthe full RF frequency depends on the particular FEL. It takes 6.1milliseconds for the light beam 15 from the primary mirror beam director14 to transit the 2000 km−160 km=1,840 km of the distance in LEO at thezenith and up to 8.7 milliseconds if the light beam 15 is at a 45° angleto the zenith. If all of the electron bunches in the FEL 12 are allfilled, then there are 6.5E6 bunches of light in the light beam 15 goingthrough LEO at 45° to the zenith. A small percentage of the lightpulses, for example 1-10% of the light pulses, can be turned off in acontrolled fashion by not injecting or otherwise modulating 1-10% of theelectron bunches in the FEL or other means. Typically, this is done bymodulating the light output of the drive laser for the electron source.In the example embodiment, the pattern can be repeated every 20milliseconds or longer, where the 20 milliseconds number is picked to beslightly more than twice the longest LEO transit time of the light beam15. The result is that there would be 1.5E5 missing light pulses in the1.5E7 available pulses during the 20 milliseconds. The distribution ofthese missing pulses may be varied such that they produce detailedinformation on the location of the OD 16 for the light reflected 17 fromthe OD 16 by monitoring the time-of-flight (TOF) of the light pulses inthe light beam 15, from (a) when they leave the primary mirror beamdirector 14, until (b) they are observed at an observation mirrortelescope 18 from reflections from the OD 16. The distance between twolight pulses in the light beam 15 is typically 0.4 meters and will bemultiples of this value if there are missing light pulses. This meansthat the distance of the OD 16 to the primary mirror beam director 14and observation mirror telescope 18 can be determined to at least theaccuracy of about 0.4 meters by utilizing TOF as is known in the art.Further, with the observation mirror telescope's 18 angular resolutionof 0.3 micro-rad, the translational location accuracy of the OD is 0.24m at zenith and 0.34 m at 45° to zenith for the 800 km altitude for thisexample embodiment. The general preference for the angular resolution ofthe observation mirror telescope is to match or be better than theapparent angle of the waist of the focus of the light beam 15 at thealtitude of the OD 16. Consequently, the orbital parameters of OD 16 canbe measured with excellent accuracy as OD 16 is tracked across the areaof observation. This, in turn, means that if the OD 16 is not removedduring the path on which it was first observed, the OD 16 can be locatedon a subsequent orbit by only having to scan a volume of characteristicdimensions of 10 m or less. In general, both the primary mirror beamdirector 14 and the observation mirror telescope 18 need to be able totrack the OD 16. This means that both are preferably capable of scanning+/−45° to the zenith within about 215 seconds of transit time for OD 16at 800 km. For OD at 160 km altitude the time is reduced to about 41seconds, and the time is about 580 seconds for OD at 2000 km altitude.

For the 800 km altitude demonstrative embodiment, it takes about 58milliseconds for OD to transit the light beam 15 with an average lengthof scan of about 0.43 km. The corresponding area of the scan asindicated above is about 0.15 km². Assuming that all the Category II ODis at 800 km, this results in 2.3E-10 of the Category II OD beingobserved. Consequently, the fraction of the 400,000 Category II ODobserved during the 58 milliseconds is 9.3E-5 Category II OD objectsbeing observed. This results in it taking about 621 seconds (10.3minutes) on average to observe a piece of Category II OD assumingsufficient light is detected. This rate equates to 50,800 Category II ODobjects being detected per year. Assuming that the system is on 90% ofthe time and that rotations of the Category II OD results in only 50% ofthe OD being detected, then an estimated 22,900 OD objects will bedetected per year meeting the goal as indicated in Table I. Due to thepeaking in OD 16 at the 800 km altitude, this estimate is approximatewithin +/−25% or somewhat higher.

Using the 1 MW FEL 12 of the example embodiment, 46,000 J of light beam15 energy is directed to the 0.5 km diameter area at 800 km altitudeduring the 58 milliseconds. Assuming the 0.8 transmission through theatmosphere, a reflectivity of 0.2 of the 2 cm diameter Category II OD16, then 2.6E-10 of the light beam 15 is in the reflected light 17.Assuming 0.8 transmission of the reflected light 17 back through theatmosphere, and assuming that this light is spread uniformly over anarea with a radius of 800 km on the Earth, with the 10 m diameterobservation mirror telescope 18 there is a net return of 8.0E-21 of thelight that started from the FEL 12. This equates to 3.7E-16 J of energy,or 2,300 eV. At 400 nm light in the example embodiment, this is 741photons. In embodiments of the present approach the observation mirrortelescope 18 system will have a filter to select the approximately 0.1%bandwidth of 400 nm light produced by the FEL 12. Assuming this is 90%efficient and that the photon detector is 50% efficient, then the numberof photons detected will be 334 per transit of 2 cm Category II OD at800 km. If 1 cm diameter Category II OD 16 had been considered, then 83photons would be observed for the 1 cm Category II OD 16 at the altitudeof 800 km. Additionally, these values assume a uniform distribution ofthe light beam 15 over the area and a corresponding uniform distributionof the reflected light beam 17 of the area back on Earth. Includingnon-uniform distributions would further increase the signal-to-noiseconsiderations of the detected light in the observational mirrortelescope 18.

While the number of photons detected is not large, they have distinctsignatures for OD. As indicated above, the observational mirrortelescope 18 will have the combined spatial and temporal resolution todetermine the position to approximately 0.4 m in both altitude and indirection of motion. The first signature for OD 16 is that the observedphotons are all at an altitude that varies a few 10s of meters at themost and less than a few meters in most instances. For example, at 800km the OD 16 is observed for a distance of slightly less than 0.5 km inthe example embodiment while an orbit is a distance of over 45,000 km.Even if the OD 16 orbit is somewhat elliptical, there will only be theindicated small variations in altitude for this short portion of theorbit. The second signature for OD 16 is that the photons appear as aroughly Gaussian distribution across the light beam 15 during the timeof the transit. The field-of-view of the observational mirror telescope18 is set to be slightly larger than the size of the light beam 15 sothe Gaussian distribution is approximately centered in thefield-of-view. The observational mirror telescope 18 system needs to beclose enough to the FEL 12 and primary mirror beam director 14 that thefield-of-view of the observation mirror telescope 18 overlaps with thelight beam 15 but the two systems do not need to be immediately adjacentto each other. The third signature for OD 16 is that the photons thatmatch the first two signatures also match the time distribution of lightpulses in the light beam 15 produced by the FEL 12. The fourth signatureis that the wavelength of the observed light in the observation mirrortelescope system 18 must match the wavelength of the FEL 12. There areenhancements that can be added to this technique in some embodiments.For example, the FEL 12 is capable of operating as a power amplifierdriven by a more conventional laser master oscillator. In such asituation, the wavelength of the FEL's 12 light beam 13 can be frequencymodulated in time to create an additional signal signature. The FEL 12output bandwidth of the light beam 13 is then also governed by the verynarrow master oscillator signal. Heterodyne detection may be used forlocking in on the detected photons in the observation mirror telescope18 and potentially even deriving velocity information from the OD 16.Having a heterodyne light beam 15 will provide increased signal-to-noisefor the detection of smaller OD 16 or OD 16 that is the furthest awayfrom the primary mirror beam director 14.

For locating OD 16, the primary mirror beam director is preferablycapable of altering its focal properties to control the field ofprojection of the light beam 15. As those of ordinary skill willappreciate, this can be accomplished by changing the distances of theoptical elements within the beam director or swapping the opticalelements, among other methods as may be known in the art. The same isthe case for the observation mirror telescope 18 so that itsfield-of-view has the overlap indicated with the light beam 15. Duringthe targeting period if the light beam is being spread over an angularrange to fill the scanned region. Because this spread in angle, thenon-diffraction limited portions of the light beam will contribute tothe amount of light filling the scanned region. Those of ordinary skillin the art of FELs should be familiar, FELs can deliver over 80%, andusually higher, as diffraction limited light. While the targeting of ODdoes not require diffraction limited FEL light, the targeting canutilize the diffraction limited FEL light, and ODR does require thediffraction limited FEL light.

Once the OD 16 has been detected such that its orbital parameters areknown to within about a few meters position, the optics for the focusand direction of the primary mirror beam director 14 are switched oradjusted from the broad tracking and scanning region to cover an areawith characteristic length dimension of less than 5 m as seen in FIG. 2. In some embodiments, the switching or adjustment may be made in lessthan a second, the key point being that the switching or adjustmentneeds to be fast enough to keep the OD in the field of projection of thelight beam for continued observation and this time may vary some withthe altitude of the OD. In some embodiments, two or more primary beamdirectors 14 may be utilized and a switching mirror, not shown, may beused to direct the beam from a smaller diameter beam director or beamdirectors, not shown, utilized for the initial broad angle projectionand scanning to a larger diameter beam director that can provide thetight focusing for narrow projection of the light beam 15 for the nextsteps. As used in this description, a narrow field projection, whichproduces a narrow beam, typically has the opening angle that produces adiffraction limited waist in the beam at a chosen elevation, i.e. awaist as small as possible with the only limitations being diffractionand atmospheric disturbances, if present. The smaller primary beamdirector mirror 14 may be less than 1 cm in diameter for someembodiments, though for managing the power density on the mirror, adiameter in the region of 5 to 20 cm may be preferred, especially in theembodiments where an intermediate diameter primary mirror with of thelight beam 15 is utilized to aid in locating the OD 16. In theseembodiments, the thousand-fold decrease in linear dimension translatesto a million-fold increase in intensity of the light beam 25 on the OD26. Within a second, the position is then known to a characteristiclength dimension of 0.4 m or better. By tracking the orbital debris tothis precision during the time of observation, the orbital parametersare determined, to typically include one or more of apogee, perigee,semi-major axis and its orientation, eccentricity, mean motion andperiod. For example, a CPU may be used to run one or more algorithmsthat calculate or estimate one or more of apogee, perigee, semi-majoraxis and its orientation, eccentricity, mean motion, period, among otherpotential orbital parameters as may be used in the art. It should beappreciated that suitable algorithms may be developed by those having atleast an ordinary skill in the art, and the present approach is notintended to be limited to any particular methodology for calculatingorbital parameters. There are options at this point that may be used inembodiments of the present approach: either (1) the system may furtherconcentrate the intensity of the light beam 25 on the OD 26, such as,for example, by additional adjustments in the focus of the primarymirror beam director, or (2) continue to track the OD 26, such as toeven better determine its orbit and/or improve the precision of theorbital parameters. It should be appreciated that those of ordinaryskill in the art may select either or both options, depending on theobjectives for a given embodiment. Concentrating light beam 25 intensitymay be used to target and remove OD 26, and results in a narrow fieldprojection. As should be appreciated by those having ordinary skill inthe art, narrow field projection produces a narrow beam. Narrow beamparameters depend on the particular embodiment, as well as the orbitalparameters of the targeted OD, and preferably the beam has an openingangle that produces a diffraction limited waist at a target distancerelative to the system. In an ideal scenario, the narrow beam has awaist as small as can be achieved at the OD's location (e.g., distancefrom the beam source). In some embodiments, diffraction and atmosphericdisturbances, if present, may be factored into the narrow fieldprojection as necessary. Depending on the details of the ranges involvedand orbital parameters, for example, the focus of the light beam 25 willresult in a narrow field/beam with a diffraction limited waist in thediameter of the light beam 25 at the distance of the OD 26. Typically,smaller OD 26 objects will be removed immediately, while larger OD 26objects will be removed on subsequent orbits above the surface of Earth21.

To remove the OD 26 by vaporization or orbit adjustment by preferentialEarth facing partial vaporization, the primary mirror beam director 24may be adjusted to produce a diffraction limited waist condition at thelocation of the OD 26. In the example embodiment, the 3 m primary mirrorbeam director 24 system can produce a 26 cm diameter diffraction limitedbeam waist in the light beam 25 at 800 km altitude. The primary mirrorbeam director 24 may, in some embodiments, include adaptive optics (AO)to achieve this small size utilizing feedback from the observationmirror telescope 28 system. With the FEL 22 operating at 1 MW andincluding the atmosphere transmission of 0.8, the result is 1.6 kW/cm²delivered to the OD 26 at 800 km altitude. Assuming that the OD's 26emissivity is the same as the OD's 26 absorption and ignoring radiativeand vaporization cooling, the resultant temperature of the OD 26 willreach the vaporization temperature of the OD 26 when it is heated overthe range of angles indicated from the zenith. These assumptions arereasonable for many materials utilized for satellites, and in addition,variations of up to +/−50% will not greatly affect the results forremoving the OD 26.

Vaporizing aluminum requires approximately 30 kJ/g and vaporizing ironrequired approximately 50 kJ/g to be absorbed. Assuming the detection ofthe OD occurs at zenith and is heated from zenith to 30 degrees awayfrom zenith, the flux from the light beam 25 drops from 1.6 kW/cm² to1.2 kW/cm² for the altitude of 800 km in the illustrative example. Usingthe average and the transit time of 62 seconds minus 2 seconds fordiscovery and fine location, and the 0.2 reflectivity of the OD 26, anet 65 kJ/cm² is absorbed by the OD 26. If this is insufficient tovaporize the OD 26, the heating of the OD 26 will produce black bodyradiation, and some embodiments of the present invention may beconfigured to observe such radiation by the observational mirror 28system and the orbit parameters of the OD 26 will be well characterized.On a subsequent orbit when the OD 26 passes over the primary mirror beamdirector 24, the OD 26 can be heated for at least twice as long. In thissituation, as the orbit of the OD 26 is well known, OD 26 can be locatedeven further out from the 30 degrees from the zenith and heated for aperiod even more than twice as long.

Although embodiments of the present approach may be employed during anytime, it is expected that OD 26 location may be preferably performed atnight so as to optimize signal-to-noise. However, once the OD's orbit isknown, the light beam 25 can be sufficiently concentrated to observe theOD 26 in daylight, and then further concentrated to remove the OD 26.Consequently, the system 10 as shown in FIG. 1 , and system 20 as shownin FIG. 2 , can run continuously.

The parameters of the example embodiment with the OD 16 being located at800 km altitude can be adjusted to optimize for other altitudes, asshould be appreciated by those of ordinary skill in the art. Forexample, at the 404 km average altitude of the International SpaceStation (ISS), OD 16 as small as 0.5 cm diameter will generate about 397photons if the maximum range of scanning is lowered to 0.1 km. Whilethere is not a large amount of OD 16 at the altitude of 404 km, thepriority to protect the crew and resources of the ISS can justifyscanning for smaller OD 16 and removing it. Similarly, the parameters ofthe embodiment may be optimized for other altitudes. Anotheroptimization can be the wavelength chosen for the light beam 13. Ingeneral, shorter wavelengths result in smaller mirrors and longerwavelengths result in better transmission through the atmosphere. Theoptimal wavelength can be selected for the locations selected for theFEL 12 and observation mirror telescope 18 on the Earth 11. In someembodiments, the wavelength may purposely be selected to be visible tothe human eye so that people have the added protection of being aware ofthe light beam 15 in some embodiments.

The average power level of the FEL 11 light beam 12 was set to 1 MW inthe example embodiment. The average FEL 11 power can range from 100 kWfor removing some OD 16 to above 1 MW or as high as the beam generatingtechnology supports. The technology disclosed in U.S. Provisional PatentApplication No. 62/680,858, filed Jun. 5, 2018, and U.S. ProvisionalPatent Application No. 62/724,893, filed Aug. 3, 2018 (which areincorporated by reference in their entirety), describe embodiments ofmegawatt class power level FELs. The size of the FEL 11 does notnecessarily increase as the power level goes above 1 MW, but additionalpower supplies may be required and the character of the electron sourcein the FEL 11 may change. Additional discussion of these aspects forgoing above 1 MW are not discussed in this disclosure so that thisdisclosure does not become export controlled.

As one of ordinary skill in the art should appreciate, there is greatflexibility in the detailed design of the free electron laser, opticalmirror systems, optical detector systems and associated informationprocess and control systems. The examples provided serve to guide theexposition of the concepts. The methods described in the presentapproach may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. It should beappreciated that numerical values used herein may be approximate, andunless specifically stated, persons of ordinary skill in the art shouldappreciate that such values are generally not intended to be finitelimitations or definitive values. The disclosed embodiments aretherefore to be considered in all respects as illustrative and notrestrictive by the foregoing description.

Also described herein are embodiments of an MOPA FEL system thatgenerates very high average power (e.g., output photon average powerabove about 10 kW). Average power as referenced herein is determined byintegrating the total photon output energy for a time period of 1second. The average electron beam current as referenced herein used inthe MOPA FEL to generate the very high average power is determined bymeasuring the total electrical charge that is generated in the electronsource, to be discussed, that enters the booster, to be discussed, for atime period of 1 second. These averages are used to distinguish frompeak powers and peak currents associated with electron bunches to bediscussed that are typically shorter in time than one picosecond and mayeven be shorter than 100 femtoseconds in some embodiments. Embodimentsdisclosed herein provide an average electron current above about 1microamp and below about 5 mA per 10 kW of average photon beam power,and some embodiments may achieve an average electron current above about1 microamp and below 2 mA per 10 kW of average photon beam power.

FIG. 3 illustrates a MOPA FEL 30 according to an embodiment of thepresent approach. MOPA FEL 30 includes an initial portion having anelectron source 311, booster 312, electron beam merge 313, andaccelerator 314. The electron source 311 generates electron bunchestypically at an energy of about 50 to about 500 kV, though in someembodiments the energy may be outside this range. Electron sources aretypically characterized as being DC, meaning that the source operates ata fixed high voltage, or RF, meaning that the source operates at a radiofrequency, RF. In addition, sources typically operate either at roomtemperature or at superconducting temperatures. Generally, any electronsource may be used in the present approach. Ideally, though, theelectron source satisfies four conditions: 1) generates electron bunchesthat produce the required average current, 2) generates electron bunchesthat are at the frequency or subharmonic of the frequency of the booster32 and accelerator 34 frequencies, 3) preferably has a normalizedelectron beam emittance between 1 nm mrad and 4 mm mrad, and 4) produceselectron bunches with a width in time that can be tuned by the electronbeam optics to produce the width in time needed by the electron buncheswhen they reach the undulators 39 and 317 so as to optimize the overlap,to be discussed, of the electron bunches and the optical bunches in theundulators 39 and 317.

The booster 312 and accelerator 314 have radio frequency cavities whoseelectromagnetic fields accelerate, and in some embodiments alsodeaccelerate, the electron bunches from the electron source. The RFfrequencies may range from about 10 MHz to about 10 GHz, but may falloutside of this range. For an optimized very high average power MOPA FELproducing photons in the ultraviolet (UV) to infrared (IR) and, in someembodiments ranging from the extreme ultraviolet (EUV), the RFfrequencies are typically between 500 MHz and 1,500 MHz, though in someembodiments they may be outside of this range. The frequency of RF inthe booster 312 may be either the same as the frequency of theaccelerator 314, or in some embodiments the booster 312 RF may be anintegral subharmonic of the accelerator 314 RF. And in some embodiments,there may be RF supplied to the booster 312 at an integral superharmonicof the accelerator 314 for the purpose of enhancing the compression ofthe electron bunches. Depending on the embodiment, as those withordinary skill in electron sources know, the booster and accelerator RFcavities may be room temperature, may be superconducting RF (SRF), ormay be a combination of the two. The frequency of the electron bunchessupplied by the electron source 311 may be any subharmonic or multiplesubharmonics and structured patterns of the booster 312 frequency toinclude a single bunch of electrons in some embodiments up the fullfrequency of the booster 312. The pattern of the light pulses, thetravel time of the pulses and their reflections, may be used to improvesignal-to-noise ratio in detecting light reflected from the OD. One ormore patterns, and/or the frequency modulation of the light beam, may bevaried in the transmitted light. The pattern variation may be controlledby a controller, such as a computer system with a CPU programmed toperform variations. The variations may be incorporated into detectionalgorithms, such that a detection mirror may detect the variations inreflected light. The controller may then implement one or morealgorithms to improve the signal-to-noise ratio. In addition, theelectron bunches may have a macro structure frequency such that anynumber of electron bunches can form a macro pulse. When the accelerator314 is linear, it may be referred to as a linac. The energy gain of theelectrons in the booster 312 is typically about 0.5 MeV to about 10 MeV,but may fall outside of this range in some embodiments. The function ofthe booster 312 is to accelerate the electron bunches to sufficientenergy such that that they can be accelerated by the accelerator 314without intolerable degradation of beam quality before delivery to theundulators. The accelerator 314 energy may range from about 1 MeV toabout 20 GeV, but can fall outside of this range for some embodiments.The energy of the accelerator 314 may be chosen to produce photon beamsof required wavelength as those of ordinary skill in FELs are aware. Asexamples, electron beam energy is typically in the 20 to 200 MeV rangefor infrared photon beams and the electron beam energy is typically inthe 2 to 20 GeV range for x-ray beams. In some embodiments utilizingSRF, the booster 312 may be included in the initial section of theaccelerator in a combined cryomodule, where the cryomodule is theoverall vacuum and cryogenic container of the SRF cavities.

Most work on very high average power FELs in the range from EUV to IRhas involved three FEL concepts but each has run into limitations: 1)OSC FELs require mirrors that typically must reflect five to twentytimes the optical power that is extracted from the undulator. Forexample when the Jefferson Lab FEL delivered 14 kW extracted beam power,there was 140 kW of optical beam power in the optical cavity. Whenextracted beam powers reach over 200 kW there is currently no mirrortechnology to contain the OSC optical beam power and have it interactwith the electron beam in the undulator, and achieve extraction of adiffraction limited beam. There may be ways to partially address thisissue with very short Rayleigh range optics but this option has not beenfully explored or demonstrated. 2) SASE FELs lose the overlap of theelectron beam and the optical beam bunches due to the slippage thatoccurs from the electrons having a longer path length in the undulatorand from traveling below the speed of light. This can be overcome if thebunch charges are high enough but this means bunch charges must be veryhigh. For example, typically nanocoulomb level bunch would be requiredto achieve very high average powers. While nanocoulomb bunches have beenproduced at relatively low repetition rates, e.g. kHz scale, they havenot been published as being produced at hundreds of MHz for sustainedperiods of time, e.g. minutes, hours, etc. at the high beam qualityrequired. Additionally, the microbunching instabilities have not beenadequately understood and addressed to be able to deliver the requiredelectron beam emittances to the SASE AMP undulator, i.e., the undulatorknown in the art used in amplifier-type FELs. And, 3) the RAFELarrangement has the same problem as the SASE FEL in that the opticalpulse extracted from the AMP undulator does not sufficiently overlapwith electron bunch in the AMP undulator to get the output efficiencydesired for very high average power EUV to IR configurations.

An optimized MOPA FEL configuration can overcome the limitations of theOSC FEL, SASE FEL, and RAFEL configurations. While historically MOPA FELconfigurations have been experimented with, they did not include theoptimized configurations disclosed herein, nor the BNNT cryosorbersand/or BNNT vibration dampers described herein. The MOPA FELconfiguration utilizes two undulators where the optical beam from oneundulator feeds the second undulator. In an example embodiment, an OSCundulator, an undulator known in the art used in oscillator-type FEL,can be used to create an optical beam that is fed into an AMP undulator.While separate electron accelerators can used to drive the twoundulators, this is not required if two separate energy beams are run inone accelerator where the final two electron energies are tuned to havea tune energies that match the MOPA FEL lasing conditions separately forthe two undulators. FIG. 4 illustrates a train of electron bunchesexperiencing acceleration in the electromagnetic fields 421 of thelinac. In this embodiment three electron bunches 42 experience slightlymore electric field than the fourth electron bunch 43. The cycle thenrepeats. The ratio of more accelerated electron bunches 42 to lessaccelerated electron bunches 43 can be changed, typically in the ratioof about 1:1 to about 1:10, but may be beyond this range. An additionalfeature illustrated in FIG. 4 is that the less accelerated electronbunches 43 are longer in space, and consequently in time, than the moreaccelerated bunches 42. This will help the subsequent optical beambunches to having different lengths in time. A further feature, notillustrated, is that amount of charge in the bunches can be different,i.e. the more accelerated bunches 42 can have more, less or the samecharge as the less accelerated bunches 43. Modern electron sourcetechnology can achieve these variations and multiple other embodimentsare apparent to those skilled in the art and are thereby included in theconsiderations.

For the optimized MOPA FEL the electron beam is directed from the linac314 (referring to the FIG. 3 embodiment) via a magnet beam line or arc318 to the AMP undulator 39. In preferred embodiments, the electron beammay initially go to the AMP undulator 39, as this is where the greatestamount of optical power is generated and the electron beam needs minimaldisturbance for optimal performance. Those electron bunches that havetheir energy tuned to the FEL conditions of the AMP undulator lase andproduce the output optical beam 311 that then illuminates the beamdirector 312 to create the optical beam from the system 313. Followingthe AMP undulator, the electron beam is circulated back 314 to the OSCundulator 317 via a vertical dogleg 315 that raises or lowers theelevation of the electron beam and feeds it into the electron beam line316 that feeds the OSC undulator. This changing elevation is labeled aPAPERCLIP MOPA FEL. In some embodiments the elevation 315 is not changedbut this may not be optimal as the crossing beamlines in the same planecan create disturbances in the electron beam. The electron bunches 423with tune energy for the OSC undulator 317 produce an optical beam 318and 321 that resonates between the optical mirrors 319 and 320. Theoutput mirror 320 extracts a fraction of the optical power in the OSCundulator 317 and this power is reflected by mirrors 322 into the AMPundulator 39. The electron beam in the OSC undulator 317 is directedback to the plane of the electron beam in the linac 314 by a verticaldecline 323 and the return arc 324. The electron bunches then merge 313with the electron bunches coming from the booster 312 to be subsequentlydeaccelerated in the linac 314, and then extracted by beam line elements315 to enter 316 the beam stop 317. An alternative embodiment of theMOPA FEL is to have a single racetrack where the linac is followed bythe AMP and OSC undulators and then the electron beam is brought back tothe linac for energy recover of electron beam. While this geometryworks, it has the disadvantage of having a larger footprint. PAPERCLIPvery high average power MOPA FELs can be designed in detail to fitwithin a 10 m wide by 40 m long (IR), 55 m long (UV), 80 m (EUV) footprint or in some embodiments larger, while a racetrack MOPA FEL wouldneed to be from 10 to 20 meters longer in most embodiments, andsometimes outside this range in some embodiments. As those of ordinaryskill in FELs are aware, there is some flexibility in the overall MOPAFEL layouts but when configured to be very high average power, there aregeometrical constraints on the beam lines transporting the electronbeams such that the electron beams have the required properties whenentering the undulators, linac and beam dump while minimizing the CSRand TR.

The importance of the different lengths of the electron bunches 42 and43 is illustrated in FIG. 5 for the AMP undulator. The lengths of theelectron bunch 51 and optical bunch 52 and 533 remain fixed. The opticalbunch length 52 is set by the electron bunches in OSC undulator 317. Theelectron bunch length 51 in the AMP undulator 39 is set by the length ofthe electron bunches 42 that are tuned to the energy of the AMPundulator 39. When the electron 51 and optical bunches 52 enter the AMPundulator 317, at its upstream end 310, the leading edges of the twobunches 51 and 52 are the same 54. Midway in the AMP undulator 39 theleading edge of the optical bunch 52 is in front of the electron bunch51. At the exit of the AMP undulator, the optical bunch 52 has slippedsufficiently along the electron bunch 51 that the overlap becomesinsufficient to generate significant additional optical beam power inthe optical beam 311.

One important consideration for the electron bunches entering the AMPundulator 39 is that they appropriately overlap in space and time withthe photon bunches that are generated in the AMP undulator 39. This isachieved in the example embodiment by limiting the degradation ofelectron bunch quality and brightness by collective effects—includinglongitudinal space charge (LSC), coherent synchrotron radiation (CSR),and a related phenomenon which results from the interaction ofstatistical density fluctuations within the bunch through LSC and CSR,the microbunching instability, during formation, acceleration, andtransport of the electron beam to both undulators. Recent advances inbeam dynamics establish methods to assess the impact of, and to control,all such effects. If the transport design is not appropriately optimizedsuch that these collective effects are mitigated the light output 313 ofthe AMP undulator 39 is greatly reduced.

In the OSC undulator 317 the length of the optical bunch is driven bythe length of the electron bunch at that location in the overall MOPAFEL 30. An important consideration in both the OSC undulator 317 and theAMP undulator 39 is that the transverse dimensions of the electronbunches and photon bunches are close to each other to optimize thelasing conversion of electron energy to photons.

At the exit of the AMP undulator 39 the portion of the electron beamwith its initial energy set for the lasing conditions of the AMPundulator have an electron beam going into the return arc 314 with anenergy distribution illustrated in FIG. 6 . Approximately two-thirds ofthese electrons convert energy into the output beam 311 and end up withan energy 61, typically 2 to 10 percent below the energy 62 of theelectron incident on the OSC undulator (e.g., that was tuned asdiscussed above), although the loss may be beyond this range in someembodiments. Approximately one-third of the electrons in this beamretain energies close to the energy entering the OSC undulator.Additionally, a small amount of electrons lose additional energyresulting in a tail 63 typically down to about 3 to about 5 times theenergy difference between the peaks 61 and 62 but sometimes beyond thisrange, and additionally a small amount of electrons increase theirenergy to up to about 2 percent above 64 the ingoing electron beamenergy. Small amount of electrons means typically less than about 5percent of the incident electrons but it may sometimes be beyond thisrange.

Due to the energy spread of the electron bunches as illustrated in FIG.6 the electron beam optics in the transport arcs 314 and 316 andpossible elevation change 315 may be configured to transport the broadrange of energies without generating levels of CSR that would degradethe beam quality going into the OSC undulator 317. Several options areavailable for the electron beam in the OSC undulator: 1) A fraction ofthe electron bunches, as discussed above, have an energy sufficientlyseparated from the electron beam tune energy of the AMP undulator 39,and this fraction drives the production of light in the OSC undulator.For a given undulator there is a range of electron beam energy that willproduce lasing. For undulators and electron beam energies producinglight in the EUV to IR the range or bandwidth of energies is typicallyfrom 1% to 0.01% of the electron beam energy though in some embodimentsthe range of energies may be broader. Sufficiently separated means thatthe tune energies of the AMP undulator 39 and the OSC undulator 317 mustbe separated by more than sum of the ranges of the two lasing energywidths of the two undulators. The fraction of electron bunches withtheir energy separated do not lase in the AMP undulator 39 but will lasein the OSC undulator 317. 2) The second option is to have all of thebeam at the tune energy going into the AMP undulator 39 and use theapproximately one-third portion of the electron beam that is minimallyaffected 62 by the AMP undulator 39 as illustrated in FIG. 6 . Thechallenge is that while the average energy of this portion of the beamis still at the incident electron beam energy, it has been spread inenergy and if the spread in energy is too high then it will not havesufficient electron current within the bandwidth or range required tolase in the OSC undulator 317. For a given embodiment, optimizing thechoice between option 1) and 2) depends on the specific embodiment, andmay be based on making a detailed analysis of the bandwidths, relativeoptical power levels selected in the AMP undulator 39 and OSC undulator.3) A transverse acceleration or kick to the electron bunches tuned infrequency to apply differently to electron bunches going to the AMPundulator and OSC undulator can be provided by a RF separator system asthose of ordinary skill in the art of transporting electron beambunches. The kick can be adjusted so that one of the resulting trains ofpulses lases in one undulator and the other train of pulses lases in theother undulator as those of ordinary skill in the art of lasing in FELswill be aware.

The AMP undulator 39 and OSC undulator 317 for embodiments of the MOPAFEL 30 may have a static spatially varying magnetic field characterizedby an average variation length λ_(μ) and an average strength K. Thevalue of λ_(μ) typically varies from about 1 mm to about 1 m but mayfall outside of this range for some embodiments. For EUV to IR the valueof λ_(μ) typically is in the range of 1 cm to 5 cm. The strength of Kvaries from 0.1 to about 3, but also may be outside of this range insome embodiments. The undulator magnetic field variation in mostembodiments is arranged to be planar or helical. The AMP undulator 39becomes a tapered undulator when for the length or a portion of the AMPundulator the value of λ_(μ) decreases or the value of K decreases, orboth λ_(μ) and K decrease, or if λ_(μ) decreases and K increases, or ifλ_(μ) increases and K decreases. The undulator equation is used todetermine the parameters:λ=λ_(μ)(1+K ²/2)/2/γ²where λ is the wavelength of the photon, λ_(μ) is the local undulatorperiod length, K is the local dimensionless undulator magnetic fieldparameter, and γ is the Lorentz factor of the electron. The slippage inthe AMP undulator 39 of the electron bunch 51 relative to the photonbunch 52 as illustrated in FIG. 5 is impacted by the tapering. Forexample, undulators referred to a tapered undulators typically have theinitial one-quarter to one-half of the tapered undulator actually nottapered as tapering is not as important when the electrons are losingrelatively lower amount of energy in the initial stages of the undulatorcompared to the rate of energy loss in the latter stages of theundulator. The amount of tapering also impacts the energy distributionof the electron beam exiting the undulator as illustrated in FIG. 6 .

When electron bunches 51 in the electron beam enter the undulator, theelectrons experience transverse accelerations and consequently radiatephotons that become the photon beams 311 and 321. Feedback between theelectron distribution and the combined optical and magnetic field causesthe electrons to bunch at the optical wavelength determined by a Lorentzcontraction and a Doppler shift. Since bunches spaced at the opticalwavelength are oscillating in synchrony the output becomes coherent,i.e. it is all of one phase and narrow in bandwidth. Typical embodimentshave output wavelengths ranging from about EUV to IR, though the photonwavelength may be outside of this range for some embodiments. Typicalembodiments may have photon bandwidths ranging from 0.0001% to 10%,though the bandwidth may be outside of this range for some embodiments.Efficient very high average power in the MOPA FEL 30 may be achieved byoptimizing two competing conditions: shorter electron bunches generatehigher peak currents, and consequently more photons. However, if theelectron bunches are too short, the photons in photon bunches producedby the electron bunches are traveling at the speed of light and they getin front of the electron bunches. The electron bunches are traveling atslightly lower speed and more importantly, the electrons in the electronbunches are taking a longer path due the presence of the magnetic fieldsof the undulators 39 and 317 that affect the path length of electronsbut do not affect the photons. As those of ordinary skill in FELs know,the length of the electron bunch is controlled by the length of theelectron bunch coming out of the electron source 311, the RF fields inthe booster 312 and accelerator 314, and the electron beam optical pathof the electrons as they enter the AMP undulator 39 and OSC undulator317. As those of ordinary skill should appreciate, the optimization ofthe competing conditions are different for high average photon beampower and high peak photon beam power. Typically, a detailed numericalsimulation is useful for optimization of a specific embodiment.

After exiting the OSC undulator 317, the post undulator electron beam324 is directed away from the photon beam 318. In some embodiments, thisis performed by placing a magnet, not illustrated, that directs the postundulator electron beam to a beam dump, not illustrated that hassufficient material to create an electromagnetic shower and sufficientcooling to absorb the remaining electron beam energy. However, thisembodiment is usually not preferred as it must deal with the significantelectrical energy inefficiency and the nuclear radiation generated bythe electromagnet shower from electrons that are above 10 MeV.

The preferred method of managing the post OSC undulator 317 electronbeam is to redirect the spent electron beam as illustrated in FIG. 3back to the linac 314 via an elevation adjusting beam line 323 andreturn arc 324 back to the merge 313. In this embodiment, the spentelectron beam exiting the undulator 317 will have an energy spectrumranging from up to about 2%, and in some embodiments higher that 2%,above the initial electron beam energy going into the OSC undulator 317as illustrated in FIG. 6 , to as low as five times the average energyloss of the beam in the OSC undulator 317. For example, if the averageelectron beam energy loss in the undulator is 10%, then some of theelectron beam will have energy as low as 50%, or slightly more, belowthe incident electron beam energy. The MOPA FEL 30 arrangement shown inFIG. 3 is known by those of ordinary skill as an energy recovery linac(ERL) arrangement. The spent electron beam is injected back into thelinac 314 via the merge elements 313 to achieve energy recovery. As thespent electron beam has a broad energy distribution that may be as muchas 50% of the initial electron beam energy, the beam line elements inthe final return arc 324 create a time energy correlation in the spentelectron beam bunches such that when they arrive at the linac 314 theelectrons with the highest (lowest) energy experience the maximum(minimum) deacceleration in the RF fields. The beam transport systemreturning the FEL exhaust beams to the linac for energy recovery are tobe configured with specific choices of linear and nonlinear momentumcompactions so as to establish—for each exhaust beam—the appropriatetime-energy correlations necessary so as to insure each time slice ofeach beam is properly synchronized to the RF waveform such that theenergy of each distinct time slice correlates to the deceleratinggradient in the linac so as to compress the exhaust energy spread duringenergy recovery.

In preferred embodiments, the spent and now deaccelerated electronbunches exiting the linac 314 and being deflected into the beam stop 317as described above need to be below the photo- or electro-neutronproduction threshold (near 10 MeV for most materials). Keeping theseelectrons below 10 MeV or slightly above this value in most embodimentsminimizes the production of neutrons and consequently reduces theshielding requirements for the beam stop, sometimes called a beam dump,317. The energy deposited in the beam dump can be converted toelectrical power by letting the beam dump core temperate to raise to the100° C.-1000° C. range, and in some embodiments above 1000° C., andconverting the thermal energy to electric power. In some embodiments,the beam dump is partially a beam stop in that the beam passes through acopper cavity to extract some of the electron energy directly as RFpower that is then returned to the linac 314 or booster 312.Consequently, beam stop technology can produce significantly reducedlevels of radioactive materials as compared to beam dump technology andcan be slightly more efficient due the direct conversion of spentelectron bunch energy to RF power. Beam dump technology may be preferredin some embodiments but the issue of possible significant residualradioactivity should be addressed. Those of ordinary skill in the artshould be familiar with assessing radioactivity for a particularembodiment.

High average power extraction in FELs requires high peak currents forthe electron beam. Such high peak currents emit CSR radiation goingaround any bend of trajectory. CSR should be avoided or minimized whenpossible as it can adversely modify the length and energy distributionof the electrons in the electron bunches and in some embodiments removesufficient power from the electron beam that supplemental thermalcooling of the beam line and its elements is required. However, someconfigurations require some bends to meet physical layout limitationsand to provide proper control systems for control of the electron beamitself and the shape including length of the electron bunches asdiscussed above. In such situations, care should be exercised in thedesign of the bend to prevent the emitted CSR from feeding back on theelectron beam and increasing its energy spread and decreasing transversebeam quality while at the same time decreasing the electron beam energy.As those of ordinary skill in the management of electron beams for FELsare aware, the generation transition radiation (TR) must also beconsidered. In an FEL TR can be generated whenever the electron bunchesexperience a change in the electrical fields typically from the imagecharges generated by currents flowing in the metal walls of the vacuumtubing surrounding the electron beam. For an efficient FEL, generationof TR needs to be minimized because it can diminish the energy andunfavorably distort the shape of the electron bunches. Additionally, TRcan heat FEL components and thereby require the inefficiency of extracooling and additional RF power to make up the energy loss. Usually thevacuum tubing diameters are kept as large as possible and transitions indiameters are made as long in length as possible to minimize the effectsof TR, but spacings and diameters required for the undulators and SRFcavities place certain limitations on required distances. The MOPA FELsystems described herein are more efficient than prior FELs as theyrequire significantly less average electron beam current to generate therequired average beam power and consequently will generate less CSR andTR for a required amount of average photon beam power.

FIG. 8 illustrates an embodiment of an MOPA FEL 80, to be referred to asa Compact MOPA FEL, according to an embodiment of the present approach.It should be appreciated that the Compact MOPA FEL may be used as analternative to a PAPERCLIP MOPA FEL, such as the PAPERCLIP MOPA FEL 30described in connection with FIG. 3 , above. Embodiments of the CompactMOPA FEL 80 may include many of the elements and parameters described inconnection with the PAPERCLIP MOPA FEL 30, such as, e.g., electronsource 81, booster 82, beam merge 83, and accelerator (referred to asthe Primary Accelerator (PA) 84 for the Compact MOPA FEL embodiment). Aunique aspect of Compact MOPA FEL 80 is illustrated in FIG. 9 . TakingfACC to represent the frequency of the accelerator 84, the electronsource 81 and electron booster 82 generate two electron bunch trains,IAMP and IOSC, at maximum frequencies fAMPmax and fOSCmax wherefAMPmax=fOSCmax=fACC/2. The frequencies of the IAMP and IACC electronbunch trains fAMP and fOSC may be any sub harmonic of fAMPmax andfOSCmax up to and including the frequencies fAMPmax and fOSCmax. FIG. 9illustrates the electron bunches placed near the peaks of theaccelerating field 91 in the accelerator 84 where in the exampleillustrated fAMP=fAMPmax and fOSC=fOSCmax/3 for the fAMP bunches 92 andthe fOSC bunches 93 respectively. As discussed for the ODR, the electronbunch charges can be varied electron bunch to electron bunch includinghaving bunches with no charge to create patterns in the flow of bunches,the average electron currents can be different for the two electronbunch trains, and the electron source can create frequency modulatedvariations in the source energy.

In the embodiment shown, the IAMP and IOSC beams exit the PA 84, areseparated at beam splitter 85 from the spent beam 87, and are circulatedto the combiner 89 via an arc 86. The combiner feeds the IAMP and IOSCbeams into the Secondary Accelerator (SA) 810, where, as illustrated inFIG. 9 , the IAMP electron bunches 95 are further accelerated and theIOSC electron bunches 96 are deaccelerated by the SA field 94.

Following the SA 810, the separator 811 takes the higher energy IAMPbeam via a circulation arc 824 to the AMP undulator 819. AMP undulator819 generates an output light beam 820, that may be directed by theoutput mirror 821 as an output FEL beam 822. The spent IAMP beam fromthe AMP undulator 819 circulates via an arc 825 to the combiner 89.Following the SA 810, the separator 811 takes the lower energy IOSC beamvia a circulation arc 812 to the OSC undulator 813. OSC undulator 813emits light beam 814, reflected by mirrors 815, 816, and 817 to AMPundulator 819, where light beam 818 is amplified before emission fromthe FEL 822. The spent IOSDC beam from the OSC undulator 813 circulatesvia an arc 823 to the combiner 89. The time phase of the spent IAMP andspent IOSC beam are controlled by the lengths of the circulation arcs823 and 825 so that they have a controlled phase relative to theaccelerating/deaccelerating fields in the SA such that they emerge withtheir energies including their energy spreads within a band to separateinto the circulation arc 826 on a path back to the PA for energyrecovery as an ERL. If required, additional beam manipulationaccelerating structures can be added independently on straight sectionsof individual arcs if this is determined to be beneficial to a givenembodiment. As those of ordinary skill should appreciate, this novelarrangement of these phasing relationships can be achieved for controlof the energies and energy spreads. Accordingly, embodiments of theCompact MOPA FEL provide high average output beams, and efficient energyrecovery of the electron beams.

Both the IAMP and IOSC spent electron bunch trains for the Compact MOPAFEL 80 are energy recovered in the PA and beam stop 88 in a fashionsimilar to what was described above for the PAPERCLIP MOPA FEL 30.Similarly, for the Compact MOPA FEL 80, the optical beams from the OSCundulator 813 and AMP undulator 819 are set up by the mirrors 815, 816and 817 in the same fashion as the mirrors for the PAPERCLIP MOPA FEL30. The energy spreads, control of pulse pattern and heterodyning arelikewise similar. The advantages of the Compact MOPA FEL over thePAPERCLIP MOPA FEL include: the IAMP and IOSC electron bunch trains onlypass through their respective undulators and do not have to pass throughan additional undulator and arc which enables more efficient control ofCSR and microbunching in the arcs and reduces spurious TR in theundulators; and, the layout can be designed to fit within a 6 m wide by36 m long (IR) and possibly smaller which is significantly smaller.

As one of ordinary skill in the art should be aware, a prior limitationon the performance of FELs has been the efficiency on the conversion ofaverage electron beam current to average photon beam power. As disclosedherein, this limitation is reduced by a factor of about 5 to about 20,and in some embodiments above this 20. By concurrently integratingmultiple advances as a system, efficient very high average power MOPAFELs become feasible and may be employed under the present approach. Thefirst advance is the development of high bunch charge/highbrightness/high average current electron guns such as demonstrated atCornell University where an electron source has demonstrated averagecurrents of 65 mA with bunch charges of up to 100 pC and normalizedemittances of less than or of the order of 0.3 mm-mrad. The Cornellelectron source allows for a tradeoff where the bunch charge can beincreased for a corresponding increase in emittance. The University ofWisconsin is also developing an electron source with similar electronbeam properties that operates with RF directly on the cathode. Thesecond advance is the realization a MOPA FEL that optimizes the balanceof feeding photon beam energy from the OSC undulator 317 into the AMPundulator 39. This, in turn, implies that the overlap between theelectron bunches and the optical field of the photon bunches ismaintained over an extended distance as illustrated in FIG. 5 , therebyincreasing the extraction efficiency of the electron-to-optical energytransfer.

The third advance relates to improvements in the vacuum systemassociated with DC electron sources that use photocathodes. The lifetimeof a photocathode required for a high quality FEL is limited by thequality of the vacuum around the photocathode. A new generation ofcryopumps that utilize boron nitride nanotubes (BNNTs) as the pumpingsurface are anticipated to extend the lifetimes of photocathodes by afactor of three or more as described in U.S. Provisional PatentApplication No. 62/427,583 (which is incorporated by reference in itsentirety) and International Application PCT/US2017/063752, filed Nov.29, 2017 (also incorporated by reference in its entirety). Using BNNTsas a cryosorber enables efficient cryopumps for use with FELs. Thepumping capacity of the cryopumps can be further increased by utilizingBNNTs, and in particular BNNTs that have been purified. BNNT materialpurified as described in U.S. Provisional Patent Application No.62/427,506 (which is incorporated by reference in its entirety) is anexample of purified BNNTs, which are especially useful for the presentapproach. Purified BNNTs have a larger surface area per unit mass,contributing to the pumping speed and capacity enhancements.

The fourth advance is in reducing the mechanical vibrations of the SRFcavities. These vibrations result in length variations of the SRFcavities that must be compensated for by increasing the RF powersupplied to the cavities. A factor of 10% to 50% reduction in the amountof RF power required can be achieved by reducing these vibrations. BNNTsare outstanding vibration dampers, and suitable for use in SRF cavities.For example, mats of purified and aligned BNNTs have viscoelasticproperties with tan δ greater than 0.1, and therefore can be utilized aspassive vibration damping at cryogenic temperatures, as described inInternational Patent Application No. PCT/US2018/017231, filed Feb. 7,2017 and incorporated by reference in its entirety. The viscoelasticperformance of the boron nitride nanotube mats may be enhanced when theBNNT material is purified, as described above. The viscoelasticperformance of the boron nitride nanotube mats may also be enhanced whenthe BNNT material is aligned as described in U.S. patent applicationSer. No. 15/305,994, filed Oct. 21, 2016 and incorporated by referencein its entirety. In embodiments featuring a beam stop as describedabove, the water cooling for the heated copper cavities may be at highpressure (e.g., pressure ranging from 1 to 50 MPa) and mechanicallyconnected to the SRF cavities in the accelerator and booster by RFwaveguides. Rapidly flowing cooling water in these temperature rangesthat feed electrical generation equipment produces mechanicalvibrations. The viscoelastic properties of the BNNTs extend to theindicated temperature ranges as described in International PatentApplication No. PCT/US2018/017231 (which is incorporated by reference inits entirety) and can be utilized to minimize the feedback of mechanicalvibrations to the RF system that in turn connects to the SRF cavities.The result of the combined low temperature and high temperaturemechanical vibration damping will result in a reduction of 10% to 50%and possibly beyond the range in the capital expense and operatingexpense for the RF power required.

The fifth advance is that SRF cavities with required levels ofperformance now operate at 4 K rather than 2 K. Previously, theresistive losses on the cavity walls (which scale as the gradientsquared) were so high at 4 K operating temperature that only very lowcavity gradients (˜5 MV/m or less) could be utilized resulting in anunacceptably large accelerator. Advances in the surface treatment ofniobium SRF cavities now result in quality factors exceeding 2¹⁰ at 2 Kand 4⁹ at 4 K. This enable operation at gradients exceeding 8 MV/m at 4K. In addition, substantial progress has been made in using othermaterials on the cavity surface (Nb3Sn and others) which have a naturalcapacity to operate at higher temperatures due to their higher criticaltemperature. Consequently, with either advance a helium refrigeratorthat operates at 4 K can be used. A 4 K refrigerator that meets therequirements has a capital and operating cost that is approximately afactor of two less than the 2 K refrigerator, along with a smaller size,mass, and higher reliability.

The sixth advance is the utilization of recently developed methods forthe management of collective effects such that the quality andbrightness of the electron bunches are preserved throughout the beamformation, acceleration, and transport process, such that the electronbunches entering the AMP undulator possess all properties appropriatefor the production of extremely high photon pulse energy, withconsequential high photon beam power.

The seventh advance is that two or more of the six preceding advancescan be combined under the present approach in an integrated systemoptimized for the integrated performance of an efficient very highaverage power MOPA FEL. The preferred optimizations are dependent on thewavelength of the photon beam, the required average power of the photonbeam, the optical properties of the photon beam, and the size of thelayout of the MOPA FEL. As one of ordinary skill in the art of FELs isaware, for a given embodiment only some of the six advances may beutilized.

As illustrative examples, a PAPERCLIP MOPA FEL that operates IR outputphoton wavelength and utilizes all six advances is presented andcompared to the Jefferson Lab FEL in the table below.

TABLE 1 Comparison of Contemporary FEL to MOPA FEL FEL Jefferson LabMOPA FEL Electron beam energy (MeV) 150 110-150 Average kW/mA 1.4 10-40Undulator length (m) 1.65 2 (OSC), 8 (AMP) Undulator period (cm) 5.5 3Change in AMP undulator NA 20-55* amplitude K (%) Ratio of OSC to AMP NA5-15 photon beam power *Note: The range in undulator amplitude comesfrom the optimization of the photon output.

As is seen in the table above, the optimized MOPA FEL has significantperformance enhancements over the prior best performance. Thisperformance is further demonstrated in FIG. 7 where the results oftime-dependent simulations using the MINERVA FEL code are shown.Megawatt-class performance is demonstrated over a range of parametersachievable by an optimized MOPA FEL. It should be appreciated thatdeviations from the disclosed embodiments may be made without departingfrom the present approach. For example, light beams may be redirectedfrom tertiary mirror(s), such as in high altitude or in orbit. Asanother example, the target need not be limited to debris orbiting theplanet. As a further example, the FEL may be airborne, or may be inspace, in some embodiments. Those of ordinary skill in the art may adaptthe present approach as needed.

As one of ordinary skill in the art of accelerator physics, freeelectron lasers and boron nitride nanotubes should appreciate, there isgreat flexibility in the detailed design of the accelerating subsystems,choices of: beam energy, beam current, undulator parameters, electronbeam optics, refrigeration systems, vacuum systems, radio frequencysources and systems, vibration damping systems, control systems, andphoton beam optics.

Embodiments of the present approach may include at least one computerwith a user interface and a CPU running one or more algorithms and/orapplications. For example, a computer may run a program havingmachine-readable program code for causing, when executed, the computerto perform steps, such as calculating orbital parameters as describedabove. It should be appreciated that algorithms are available in theart, for controlling light beam generation, focusing, and directing, andmay be modified for use in embodiments of the present approach.Likewise, algorithms are available that may be modified for varyingaspects of emitted light beams, such as light pulse patterns, and fordetecting reflected light using the varied aspects to improvesignal-to-noise ratio, and for determining orbital parameters andtracking identified OD. A person having ordinary skill in the art mayalso develop algorithms for performing aspects of the present approach,as such algorithms will depend heavily on the specific embodiment. Thepresent approach is not limited to any particular algorithms forperforming the calculations described herein, nor is the presentapproach limited to any particular CPU or controller(s).

The systems and methods described above are for example only, and may beimplemented, in part, in any type of computer system or programming orprocessing environment, or in a computer program, alone or inconjunction with hardware. The present approach may also be implementedin software stored on a non-transitory computer-readable medium andexecuted as a computer program on a general purpose or special purposecomputer. It should thus be understood that the present approach is notlimited to any specific computer language, program, or computer.

The examples provided serve to guide the exposition of the concepts. Themethods described in the present approach may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. It should be appreciated that numerical valuesused herein may be approximate, and unless specifically stated, personsof ordinary skill in the art should appreciate that such values aregenerally not intended to be finite limitations or definitive values.The disclosed embodiments are therefore to be considered in all respectsas illustrative and not restrictive by the foregoing description.

What is claimed is:
 1. A method for free electron laser (FEL) orbitaldebris removal, the method comprising: emitting a light beam from an FELto a beam director, the light beam having light pulses, and at least aportion of the light beam comprising a diffraction-limited portion;varying at least one of a pattern of the light pulses and a frequencymodulation of the light beam; directing the light beam from the beamdirector to a search region; detecting a reflected light beam at anobservation mirror, the reflected light beam having at least one of areflected light pulse patter and a reflected modulated frequency;identifying an orbital debris in the search region from the at least oneof a reflected light pulse pattern and a reflected modulated frequency;defining at least one orbital parameter of the orbital debris; andre-directing the light beam from the beam director at the orbital debrisusing the at least one orbital parameter, the re-directed light beamdefining a narrow field projection light beam; wherein the MOPA FELhaving at least one of a boron nitride nanotube (BNNT) cryosorber and aBNNT vibration damper.
 2. The method of claim 1, further comprisingtracking the identified orbital debris with a broad field projectionlight beam.
 3. The method of claim 1, wherein the emitted light beamcomprises a broad beam having a cone opening angle of at least 0.05milliradians and less than 2.0 milliradians.
 4. The method of claim 1,wherein the FEL comprises a MOPA FEL.
 5. The method of claim 1, whereindirecting the light beam at a search region comprising using a firstmirror having a broad field projection for scanning and tracking, andredirecting the light beam at the orbital debris comprises using asecond mirror.
 6. The method of claim 5, wherein re-directing the lightbeam at the orbital debris comprises focusing a waist of thediffraction-limited portion of the light beam at the orbital debris. 7.The method of claim 5, further comprising switching between the firstmirror and the second mirror at a rate sufficient to maintain the ODwithin a field of projection of the light beam.
 8. The method of claim4, wherein the MOPA FEL comprises: an electron source with an averageelectron beam current below about 2 mA and above 1 microamp per 10 kW ofaverage photon beam power, the electron source having an emittance lessthan 4 mm mrad and above 1 micrometer mrad, an energy spread less thanone part in one hundred, an electron booster, an electron accelerator,an OSC undulator, and an AMP undulator.
 9. The method of claim 8,wherein the MOPA FEL further comprises at least one of a boron nitridenanotube (BNNT) cryosorber and a BNNT vibration damper.
 10. The methodof claim 8, wherein the AMP undulator comprises one of a taperedundulator and a non-tapered undulator.
 11. The method of claim 8,wherein at least one of the OSC undulator and the AMP undulator is oneof a planar undulator and a helical undulator.
 12. The method of claim8, wherein the AMP undulator comprises a planar undulator having anon-tapered portion.
 13. The method of claim 8, wherein at least one ofthe OSC undulator and the AMP undulator comprises a helical undulator.14. The method of claim 8, wherein the MOPA FEL further comprises SRFcavities incorporating a boron nitride nanotube vibration damper. 15.The method of claim 8, wherein the MOPA FEL comprises a Compact MOPAFEL.
 16. The method of claim 15, wherein the emitting a light beamcomprises generating light by: splitting an accelerated into a firstsplit electron beam and a second split electron beam, passing the firstsplit electron beam through an AMP undulator, passing the second splitelectron beam through an OSC undulator generating an OSC light beam,amplifying the OSC light beam in the AMP undulator to generate theemitted light beam, and passing the first split electron beam after theAMP undulator and the second electron split beam after the OSC undulatorthrough a combiner, deaccelerating the first split electron beam andaccelerating the second split electron beam in the secondary acceleratorto form a combined electron beam, passing the combined electron beamthrough the primary accelerator for energy recovery.
 17. A free-electronlaser (FEL) orbital debris removal system comprising: a MOPA FEL havingaverage power above about 10 kW and configured to emit a light beamhaving light pulses, and at least a portion of the light beam comprisinga diffraction-limited portion; a light pulse pattern and frequencymodulation controller; a beam director configured to receive the emittedlight beam and re-direct the emitted light beam to a search region, thebeam director having a scanning mirror with a broad field projection anda removal mirror having a narrow field projection, the beam directorconfigured to switch between the scanning mirror and the removal mirror;an observation mirror configured to detect a reflected light beam fromthe search region, the reflected light beam having a reflected lightpulse pattern and reflected modulated frequency; an orbital debrisidentification controller configured to identify an orbital debris inthe search region from at least one of the reflected light pulse patterand the reflected modulated frequency, and provide at least one scanningmirror orbital debris orbital parameter, wherein the MOPA FEL comprisesa Compact MOPA FEL having an electron beam splitter, the electron beamsplitter directing a first split electron beam through an AMP undulatorand a second split electron beam through an OSC undulator, and anelectron beam combiner after the AMP undulator and the OSC undulatorwith an output electron beam arc to a primary accelerator for energyrecovery.
 18. The FEL orbital debris removal system of claim 17, whereinthe MOPA FEL has an average beam power of at least about 100 kW.
 19. TheFEL orbital debris removal system of claim 17, wherein the MOPA FEL hasan average power of at least about 1 MW.
 20. The FEL orbital debrisremoval system of claim 17, wherein the OSC undulator comprises anoutput light beam path and the AMP undulator comprises an input lightbeam path connected to the output light beam path.
 21. The FEL orbitaldebris removal system of claim 17, wherein the FEL further comprises atleast one of a boron nitride nanotube (BNNT) cryosorber, a BNNTvibration damper.
 22. A free-electron laser (FEL) orbital debris removalsystem comprising: a MOPA FEL having average power above about 10 kW andconfigured to emit a light beam having light pulses, and at least aportion of the light beam comprising a diffraction-limited portion; alight pulse pattern and frequency modulation controller; a beam directorconfigured to receive the emitted light beam and re-direct the emittedlight beam to a search region, the beam director having a scanningmirror with a broad field projection and a removal mirror having anarrow field projection, the beam director configured to switch betweenthe scanning mirror and the removal mirror; an observation mirrorconfigured to detect a reflected light beam from the search region, thereflected light beam having a reflected light pulse pattern andreflected modulated frequency; an orbital debris identificationcontroller configured to identify an orbital debris in the search regionfrom at least one of the reflected light pulse patter and the reflectedmodulated frequency, and provide at least one scanning mirror orbitaldebris orbital parameter; and at least one of a boron nitride nanotube(BNNT) cryosorber, a BNNT vibration damper.